Committed height is bollocks. (more on that)
15 seconds to go through the mantra...about right, including stabilising. Should be doable quicker at test stage.
The reason i call bollocks on committed height comes from my limited knowledge of what is taught in BPPP.
Their training on barons, is that if the flaps and wheels are not up, then it it close both throttles and land ahead. Treat the aeroplane as a single.
As for an instructor doing the simulation by a mixture cut...that will be the end of the flight for me. Its a training flight! Throttle cut, that way if i get it wrong, then either of us can bring the throttle up again. Sure still not instantaneous, but a dammed sight quicker if you are 3/4 through the drill and need power.
As for zero thrust, we all rely on our instructors to know those numbers. Mine made me learn them and correct him where required. he probably didn't need me to tell him, but me knowing them also gave a safety scope in that I knew what was needed and if outside of it i could call a halt to the exercise.